In the paper industry foam technique, where foam is used as a carrier phase of materials, has been used in both web formation and web coating processes. The technique is described e.g. in the publications Radvan, B., Gatward, A. P. J., The formation of wet-laid webs by a foaming process, Tappi, vol 55 (1972) p. 748; a report by Wiggins Teape Research and Development Ltd., New process uses foam in papermaking instead of avoiding it, Paper Trade Journal, Nov. 29, 1971; and Smith, M. K., Punton, V. W., Rixson, A. G., The structure and properties of paper formed by a foaming process, TAPPI, January 1974, Vol. 57, No 1, pp. 107-111.
In GB 1 395 757 there is described an apparatus for producing a foamed fiber dispersion for use in the manufacture of paper. A surface active agent is added to fibrous pulp with a fibre length in excess of about 3 mm, to provide a dispersion with an air content of at least 65%, to be discharged onto the forming fabric of a papermaking machine. The aim is to achieve uniform formation of the fibrous web on the fabric.
By the middle of the 1970s the foam forming process had been successfully demonstrated on a production machine. In the Wiggins Teape Radfoam process (Arjo Wiggins) fibres were delivered to the wire of a conventional Fourdrinier paper machine in suspension in aqueous foam. The development team obtained a non-layered 3D structure in papers made on a Fourdrinier machine at very high concentrations of fibres (3-5%) in water using foam.
When comparing foam and water forming methods one trend is clear. With foam forming the bulk is bigger, but the tensile index is smaller. With a bulkier structure the structure is more porous, which leads to smaller tensile index values. An interesting result from a comparison of water and foam laid samples was that tensile stiffness indexes in both cases were very close even though foam formed samples were much bulkier. The reason for that is currently unknown and requires further research.
Surfactants used in the foaming process have a negative influence on both the dry and wet tensile strength of a paper web.
The tensile strength loss may be explained by a decrease in the dry tensile strength of a paper sheet as surfactants are adsorbed on fibre surfaces hindering hydrogen bonding between the fibres. The initial wet strength is reduced by surfactants, especially for a dry content of 8-25%, due to a reduction in surface tension which results from the weakening of the main force holding the wet sheet together.
According to current understanding the main problems, which have prevented foam forming from becoming a standard web forming technology in paper, paperboard and cardboard production, are:
too high porosity in some applications,
reduced strength properties compared to normal low consistency wet forming,
inferior Scott bond,
inferior tensile strength, and
inferior elastic modulus.
With foam forming a higher bulk (lower density) can be obtained as compared to normal wet forming. For typical printing and packaging paper and board grades the main drawbacks are the loss of elastic modulus (“softness”) and internal strength (Scott bond or z-strength). However, the same characteristics are advantages in tissue making. Thus foam forming has been much more common in tissue paper products.
A more recent approach of improved papermaking, aiming at improving dewatering and retention of papermaking chemicals in a fibrous web formed on a forming fabric, is incorporation of microfibrillated cellulose (MFC) in the pulp suspension. U.S. Pat. No. 6,602,994 B1 teaches use of derivatized MFC with electrostatic or steric functionality for the goals, which even include better formation of the web. According to the reference the microfibrils have a diameter in the range of 5 to 100 nm.
However, the drawbacks experienced with MFC are densification and high drying shrinkage of the paper, as well as a tendency of MFC to absorb and retain a substantial amount of water, which increases the energy required for drying and reduces paper machine speed and productivity. For these reasons MFC has not won extensive use in paper industry so far.